As a catalyst used when acrylic acid is produced in such a manner that acrolein is subjected to catalytic gas phase oxidation with molecular oxygen, there has widely been used a catalyst containing molybdenum and vanadium as essential components (hereinafter referred to as the molybdenum-vanadium catalyst in some cases) The molybdenum-vanadium catalyst has been known to have higher activity (in which an acrolein conversion may sometimes be used in place of activity) as compared with other catalysts. In usual cases, when catalytic gas phase oxidation of acrolein is carried out using a fixed-bed catalytic reactor filled with a catalyst, since the oxidation is accompanied with heat generation, a “hot spot” (i.e., a local abnormally high temperature part) is generated in the catalyst bed. When this hot spot has a too high temperature, the occurrence of excessive oxidation allows thermal degradation of the catalyst to proceed and further induces run away reaction.
As one means of solving the above problem, various techniques have been proposed in which a catalyst bed is divided into some parts in a tube axial direction of the catalyst reactor, the activity of a catalyst to be filled in the reaction gas inlet side on which a hot spot may easily be formed being set lower than that of a catalyst to be filled in the reaction gas outlet side.
For example, there have been proposed techniques in which a catalyst having activity lowered by incorporation of an inert substance is filled in the reaction gas inlet side of a catalyst bed (e.g., JP-B 53-30688 (1978)) and techniques in which the ratio of an active component carried on a carrier to be filled in the reaction gas inlet side is lowered (e.g., JP-A 07-10802 (1995)). In addition, there have also been proposed techniques in which a catalyst having activity lowered by addition of, for example, an alkali metal is filled (e.g., JP-A 2000-336060) and techniques in which the particle diameter of a catalyst to be filled in the reaction gas inlet side is set larger than that of a catalyst to be filled in the reaction gas outlet side (e.g., JP-A 09-241209 (1997)).
In usual cases, catalytic activity is gradually lowered by thermal degradation and the like, so that the acrolein conversion is reduced when operation time (i.e., duration of oxidation) becomes longer. Therefore, in order to keep the acrolein conversion higher, it is necessary to raise the reaction temperature. However, when the reaction temperature is raised, thermal degradation of a catalyst further proceeds. Therefore, when a catalyst having lower activity is filled in the reaction gas inlet side of a catalyst bed as in the above prior art, some effect is attained for the control of a hot spot temperature, but it cannot be said that this has sufficient productivity for a long term because of a lowering of activity. As a matter of course, productivity may be increased by setting the reaction temperature higher even in the prior art. However, since some problems arise on the thermal degradation of a catalyst and the formation of a hot spot, this is also not practical.